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United States Patent |
6,163,636
|
Stentz
,   et al.
|
December 19, 2000
|
Optical communication system using multiple-order Raman amplifiers
Abstract
In accordance with the invention an optical fiber communication system
comprising a source of optical signals and an optical fiber transmission
line is provided with one or more multiple-order distributed Raman effect
amplifiers downstream of the source for amplifying the transmitted
signals. As compared with a communication system using conventional first
order Raman amplifiers, multiple-order amplifier systems can have reduced
noise, longer fiber span lengths and reduced nonlinearities. In a
preferred embodiment the system uses signal wavelengths in the range
1530-1570 nm, first order Raman pumping at 1430-1475 nm and second order
pumping at about 1345 nm. Advantageously, the second order pump light is
copropagating with the signal light and the first order pump is
counterpropagating with the signal.
Inventors:
|
Stentz; Andrew John (Clinton, NJ);
Walker; Kenneth Lee (New Providence, NJ)
|
Assignee:
|
Lucent Technologies Inc. (Murray Hill, NJ)
|
Appl. No.:
|
233318 |
Filed:
|
January 19, 1999 |
Current U.S. Class: |
385/24; 359/334; 359/341.33; 372/3; 372/6; 398/1 |
Intern'l Class: |
H01S 003/30 |
Field of Search: |
385/24
372/3,6
359/334,124,341,134
|
References Cited
U.S. Patent Documents
4616898 | Oct., 1986 | Hicks, Jr. | 385/24.
|
5623508 | Apr., 1997 | Grubb et al. | 372/3.
|
5790300 | Aug., 1998 | Zediker et al. | 359/334.
|
5832006 | Nov., 1998 | Rice et al. | 372/3.
|
5880877 | Mar., 1999 | Fermann et al. | 359/341.
|
Primary Examiner: Sanghavi; Hemang
Assistant Examiner: Kim; Ellen E.
Attorney, Agent or Firm: Matthews, Collins, Shepherd & Gould, P.A.
Claims
What is claimed is:
1. In an optical fiber communication system comprising a source of
information-carrying optical signals, an optical fiber transmission line
for carrying the optical signals, an amplifier for amplifying the optical
signals, and a receiver for detecting and demodulating the optical
signals,
the improvement wherein said amplifier comprises a multiple order Raman
effect amplifier comprising a first pump source for launching into the
optical fiber transmission line first order Raman pump light at wavelength
for amplifying the optical signals by the first order Raman effect and a
second pump source for launching into the optical fiber transmission line
second order Raman pump light at wavelength for amplifying the first order
pump light.
2. The improved communication system of claim 1 wherein said amplifier
comprises a plurality of multiple order Raman effect amplifiers
distributed along said optical fiber transmission line.
3. The improved communication system of claim 1 wherein the first order
pump light is counterpropagated in said transmission line with respect to
the optical signals.
4. The improved communication system of claim 3 wherein the first-order
pump light is at a power equal to or less than one-half the power of the
second pump light.
5. The improved communication system of claim 1 wherein the second order
pump light is counterpropagated in said transmission line with respect to
the first order pump light.
6. The improved communication system of claim 1 wherein the first order
pump light is detuned from the optical signals by one-half to three halves
of the Stokes shift and the second order pump light is detuned from the
signals by three halves to five halves of the Stokes shift.
7. The improved communication system of claim 1 wherein the center
wavelength of the second order pump is more than one Stokes shift from the
center wavelength of the first order Raman pump.
8. The improved communication system of claim 1 further comprising an
erbium-doped fiber amplifier following the multiple order Raman effect
amplifier in the transmission line.
9. The improved communication system of claim 1 further comprising a third
pump source for launching into the transmission line third order Raman
pump light for amplifying the second order pump light.
10. The improved communication system of claim 1 wherein the optical
signals have a center frequency, the second order pump has a center
frequency, and the frequency shift between the center frequency of the
signals and the center frequency of the second order pump is in the range
from 26 to 32 THz.
Description
FIELD OF THE INVENTION
This invention relates to optical communication systems and, in particular,
to optical communication systems with amplification provided by
multiple-order Raman effect amplifiers.
BACKGROUND OF THE INVENTION
Optical fiber communication systems are beginning to achieve their great
potential for the rapid transmission of vast amounts of information. In
essence, an optical fiber system comprises a source of
information-carrying optical signals, an optical fiber transmission line
for carrying the optical signals and a receiver for detecting the optical
signals and demodulating the information they carry. The signals are
typically within a wavelength range favorable for propagation within
silica fibers, and preferably comprise a plurality of wavelength distinct
channels within that range.
The optical fibers are thin strands of glass of composition capable of
transmitting optical signals over long distances with very low loss. They
are small diameter waveguides characterized by a core with a first index
of refraction surrounded by a cladding having a second (lower) index.
Light rays which impinge upon the core at an angle less than a critical
acceptance angle undergo total internal reflection within the fiber core.
These rays are guided along the fiber with low attenuation. Typical fibers
are made of high purity silica with Germania doping in the core to raise
the index of refraction. A transmission line may include many long
segments of such fiber separated by intermediate nodes for adding or
dropping off signal channels.
Despite significant progress in reducing the attenuation characteristics of
optical fibers, signals transmitted through them are attenuated by the
cumulative and combined effect of absorption and scattering. Consequently
long distance transmission requires periodic amplification.
One approach to optical amplification utilizes Raman effect amplification.
In the Raman effect, light traveling within a medium is amplified by the
presence of lower wavelength pump light traveling within the same medium.
The gain spectrum of a silica fiber pumped by a monochromatic pump was
first measured in 1972. The maximum gain occurs when the signal is at a
frequency approximately 13 THz lower than the frequency of the pump. The
frequency (or wavelength) difference between the pump and the frequency
(or wavelength) of maximum gain is often referred to as the Stokes shift,
and the amplified signal is referred to as the Stokes wave. Use of a pump
that is detuned from the signals by about one Stokes shift (1/2 the Stoke
shift to 3/2 the shift) is referred to as first-order Stokes pumping.
It has also been observed that the gain is significantly larger for a
co-polarized signal and pump. This polarization sensitivity can be
eliminated if the pump is depolarized, polarization-scrambled on a
sufficiently fast time scale or composed of two equally powerful polarized
pumps that are polarization multiplexed. See, for example, U.S. Pat. No.
4,805,977, issued to Y. Tamura et al and entitled "Optical Coupler for
Optical Direct Amplifier".
Signal amplification utilizing distributed first order Raman effect
amplifiers is described in U.S. Pat. No. 4,616,898 issued to John W.
Hicks, Jr. on Oct. 14, 1986. The Hicks et al. system disposes a plurality
of optical Raman pumps at spaced intervals along the transmission line.
These pumps inject into the optical fiber optical pump light at a
wavelength shorter than the signal wavelengths by a Stokes shift, so that
the presence of the pump light amplifies the lower wavelength signals by
the first order Raman effect.
The use of first-order Stokes pumping has several limitations. The power of
a strong Raman pump in amplifying a weak signal will always decrease
exponentially with of distance as the light propagates into the
transmission fiber. This means that regardless of how powerful the pump,
most of the amplification occurs relatively near the point where the pump
is injected into the fiber (typically within 20 km). This significantly
limits the improvement in the signal-to-noise ratio that the Raman pump
can induce. As the pump power is increased, Rayleigh scattering of the
signal limits the improvement in the signal-to-noise ratio.
In some systems, the distributed Raman amplifiers may be followed by erbium
amplifiers. The increased signal power at the input of the erbium
amplifier causes the erbium amplifier to have a higher noise figure than
it would in the absence of distributed Raman amplification. This effect
increases the noise figure of the composite erbium/Raman amplifier and
therefore decreases the improvement in the signal-to-noise ratio.
SUMMARY OF THE INVENTION
In accordance with the invention an optical fiber communication system
comprising a source of optical signals and an optical fiber transmission
line is provided with one or more multiple-order distributed Raman effect
amplifiers downstream of the source for amplifying the transmitted
signals. As compared with a communication system using conventional first
order Raman amplifiers, multiple-order amplifier systems can have reduced
noise, longer fiber span lengths and reduced nonlinearities. In a
preferred embodiment the system uses signal wavelengths in the range
1530-1570 nm, first order Raman pumping at 1430-1475 nm and second order
pumping at about 1345 nm. Advantageously, the second order pump light is
copropagating with the signal light and the first order pump is
counterpropagating with the signal.
BRIEF DESCRIPTION OF THE DRAWINGS
The advantages, nature and various additional features of the invention
will appear more fully upon consideration of the illustrative embodiments
now to be described in detail in connection with the accompanying
drawings. In the drawings:
FIG. 1 is a schematic diagram of an optical fiber communication system
using multiple order Raman amplifiers;
FIG. 2 illustrates a typical multiple order Raman amplifier;
FIGS. 3A, 3B and 3C are graphical illustrations showing the evolution of
communication signal power in three different Raman amplification
arrangements;
FIGS. 4A, 4B and 4C are graphical illustrations showing the equivalent gain
and equivalent noise figures for the three different arrangements plotted
in FIG. 3;
FIG. 5 is a qualitative spectral diagram showing how the center wavelength
of the second-order Raman pump can be shifted to compensate gain tilt;
FIG. 6 is a qualitative diagram showing how the spectral distribution of
power can be shaped to flatten signal gain;
FIG. 7 illustrates an alternative embodiment of a multiple order Raman
amplifier;
FIG. 8 is a graphical illustration showing the evolution of communication
signal power in a system using the amplifier of FIG. 7; and
FIG. 9 shows an alternative Raman amplifier.
It is to be understood that these drawings are for purposes of illustrating
the concepts of the invention and, except for the quantitative graphs, are
not to scale.
DETAILED DESCRIPTION
Referring to the drawings, FIG. 1 schematically illustrates an optical
fiber communication system 9 using one or more distributed multiple-order
Raman amplifiers 10. The communication system 9 comprises an optical
source 11 of information-carrying optical signals and an optical fiber
transmission line 12 for carrying the signals to at least one optical
receiver 13. The line 12 can comprise a plurality of optical fiber
segments 12A, 12B, . . . interconnected at a plurality of nodes 14A, 14B,
. . . . Typically signal channels can be added or dropped at the nodes.
The Raman amplifiers 10 may optionally be followed by rare earth doped
fiber amplifiers 15, such as erbium doped amplifiers.
The source 11 can comprise a modulated laser or light-emitting diode.
Preferably it is an array of lasers or diodes for providing a plurality of
modulated wavelength distinct optical signals for a wavelength division
multiplexed (WDM) optical system. The signals can be modulated as by pulse
position modulation or pulse duration modulation.
The transmission line 12 can comprise one or more segments of
telecommunications fiber, and the nodes can be any one of a variety of
add/drop nodes known in the art for WDM systems. Disposed along line 12
downstream from the signal source are a plurality of multiple-order Raman
amplifiers 10 distributed along the length of line 12. The amplifiers 10
are preferably disposed intermediate ends of continuous fiber segments.
FIG. 2 illustrates a typical multiple order Raman amplifier 10 comprising
one or more first order Raman pump sources 20A and one or more second
order Raman pump sources 20B. The sources are typically semiconductor
lasers and are coupled together, as by a coupler line segment 21. They are
coupled into the fiber transmission line 12 as by a
wavelength-division-multiplexer 22. Advantageously, the first order pump
.lambda..sub.p1 light is counterpropagated with respect to the
communication signal light .lambda..sub.s in line 12 to reduce
pump-mediated cross talk. The pump sources 20A and 20B are preferably
depolarized, polarization scrambled, or polarization multiplexed to
minimize polarization sensitivity of the Raman gain.
The multiple order amplifiers 10 inject both first- and second-order
Stokes-shifted pumps into the transmission fiber. A first-order Raman pump
light .lambda..sub.p1 is detuned from the signals by one half to three
halves of the Stokes shift, and a second-order pump light .lambda..sub.p2
detuned from the signals by three halves to five halves of the Stokes
shift. The second-order pump amplifies the first-order pump in the
transmission fiber, and the first-order pump amplifies the communication
signal.
Where, as here, both the first-order and second-order pump powers are
injected at the same point, it is advantageous to have lower launched
power in the first-order pump than the second-order pump to prevent rapid
depletion of the second-order pump. Preferably the first-order pump power
is half the second-order pump or less. FIGS. 3A, 3B and 3C show power
versus distance for .lambda..sub.p2 =1366 nm, .lambda..sub.p1 =1445 nm and
.lambda..sub.s =1555 nm, respectively. Each plot shows curves for the
following cases: (a) solely a first-order pump, (b) a first- and
second-order pump with equal launch powers, and (c) a first-order pump
with 10 mW of power and the rest of the power in the second-order pump.
Note that the first-order pump reaches its maximum power at a significant
distance (comparable to the Beers length) within the transmission fiber.
The invention can now be better understood by consideration of the
following specific example:
EXAMPLE 1
In an optical fiber communication system, the first-order pump has
wavelengths ranging from 1430-1475 nm and is generated by sets of
polarization multiplexed diodes. The total power in this wavelength range
is less than 100 mW. The second-order pump is at approximately 1345 nm
with an output power of 400 mW. The first- and second-order pumps are
wavelength-division-multiplexed and injected into the transmission fiber
in the direction opposite of the direction of propagation of the signals.
Signal wavelengths fall within the range of 1530-1570 nm. The transmission
fiber is composed of 80 km of nonzero-dispersion-shifted fiber with an
effective area of approximately 55 square microns.
By pushing the gain experienced by the signal wavelengths further into the
transmission, the equivalent noise figure generated by a second-order
Raman pump can be lower than that of a first-order Raman pump. This effect
is demonstrated in FIGS. 4A, 4B and 4C. FIG. 4A is a plot of gain versus
total power for the following cases: (a) solely first order pump, (b)
first and second order pump with equal launch powers and (c) a first order
pump with 10 mW of power and the rest of the power in the second order
pump. FIGS. 4B is a plot of the equivalent noise figure versus total power
for the same cases, and FIG. 4C plots noise figure versus gain. For
equivalent gain, the case of second-order pumping always yields a lower
equivalent noise figure, and for pump powers where the equivalent noise
figures are equal, the equivalent gain for the case of second-order
pumping is lower. Under these conditions, the signal powers reaching the
input of an erbium amplifier are lower with second-order Raman pumping
than with first-order Raman pumping, causing a smaller increase in the
noise figure of the erbium amplifier by the addition of distributed Raman
amplification.
By selecting the center wavelength of the second-order Raman pump to be
greater than one Stokes shift from the center wavelength of the
first-order Raman pump, the second-order pump can be used to help
compensate for the gain tilt induced when the shorter wavelengths amplify
the longer wavelengths of the first-order pump. This effect is illustrated
in FIG. 5. The wavelength distribution of power within each of the pumps
may be shaped in order to generate a broad and flat signal gain as
illustrated in FIG. 6.
FIG. 7 illustrates an alternative multiple order Raman amplifying
arrangement wherein the first and second order pump sources 20A and 20B
are spaced apart along the length of line 12 and the pumps are
counterpropagated along line 12. Here light from the first order Raman
pump source 20A is injected into line 12 in the direction
counterpropagating to the communication signals, and light from the second
order Raman pump source 20B is injected in the direction co-propagating
with the signals.
The advantage of the FIG. 7 arrangement is that the signals experience
little Raman gain from the co-propagating second-order Raman pump due to
the large frequency difference between the signals and the second-order
pump. Therefore little noise from the second-order pump is transferred to
the signals. Nonetheless, the counterpropagating first order pump does
experience substantial amplification from the second-order pump in the
beginning of the transmission span. Using this geometry, significant Raman
gain for the signals can be achieved throughout the transmission span,
thereby minimizing power excursions of the signals. By minimizing the
power excursions of the signal, the system impairments due to optical
nonlinearities are reduced. To minimize the transfer noise from the second
order pump to the communication signals, the frequency shift between the
center frequency of the signals and the center frequency of the
second-order pump should fall in the range from 26 to 32 THz.
This embodiment may be better understood by consideration of the following
specific example.
EXAMPLE 2
In a communication system, the first-order pump has wavelengths ranging
from 1430-1475 nm generated by sets of polarization-multiplexed diodes.
The total power in this wavelength range is approximately 300 mW. The
second-order pump at approximately 1345 nm with an output power of 400 mW.
The first-order pump is injected into the transmission fiber in a
direction counter-propagating to the signal. The second-order pump is
injected into the transmission fiber in a direction co-propagating with
the signal. Signal wavelengths fall within the range of 1530-1570 nm. The
transmission fiber is composed of 40 km of non-zero dispersion-shifted
with an effective area of approximately 55 square microns.
FIG. 8 is a graphical illustration showing the evolution of communication
signal power in the system of Example 2.
FIG. 9 shows another alternative embodiment wherein the use of
multiple-order Raman pumps is taken one step further by injecting first-,
second- and third-order pumps (20A, 20B and 20C, respectively) into the
transmission fiber. If the first-and second-order pumps are less powerful
than the third-order pump, the position of the peak Raman gain experienced
by the signal is pushed further into the transmission system, further
improving the signal-to-noise figure. In a typical communications system
with signals near 1550 nm, the center wavelengths of the first, second-and
third order pumps are approximately .lambda..sub.p1 =1450, .lambda..sub.p2
=1360 and .lambda..sub.3 =1290 nm. Advantageously, if the pump near 1450
nm has a large bandwidth, the amplification of the long wavelength
spectrum near 1450 nm by the short wavelength spectrum near 1450 nm could
be compensated by shifting the second-and third-order Stokes pumps to
shorter wavelengths. For instance, the first, second-and third-order pumps
can be 1430-1475 nm, 1345 nm and 1270 nm. The application of even higher
order Raman pumping is possible but limited by the increased loss of
standard transmission fiber at shorter wavelengths.
It is to be understood that the above-described embodiments are
illustrative of only a few of the many possible specific embodiments which
can represent applications of the principles of the invention. Numerous
and varied other arrangements can be readily devised by those skilled in
the art without departing from the spirit and scope of the invention.
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